A power amplification circuit in a wireless system is typically a large signal device. In wireless local area network (WLAN) systems, the power amplifier circuit may transmit output signals at average power levels in the range of 10 dBm to 15 dBm, and peak power levels of about 25 dBm, for example. In WLAN systems, which use OFDM or CCK modulation, output power levels may vary widely such that the ratio of the peak power level to the average power level may be large, for example, 12 dB for OFDM and 6 dB for CCK. Because of these large swings in output power levels, power amplifier (PA) circuits may distort the output signal. Distortion, however, is a characteristic, which may be observed in PA circuits that are utilized across a wide range of applications, and may not be limited to PA circuits utilized in wireless systems. There are two metrics, which may be utilized to evaluate the distortion performance of PA circuits. These metrics may be referred to as amplitude modulation to amplitude modulation (AM-AM) distortion, and amplitude modulation to phase modulation (AM-PM) distortion.
The AM-AM distortion provides a measure of the output power level, pout, in response to the input power level, pin. The input power level, and output power level are each typically measured in units of dBm, for example. In an ideal, non-distorting, PA circuit, the output power level changes linearly in response to a change in the input power level. Thus, for each Δpin change in the input power level there may be a corresponding change in the output power level Δpout. The AM-AM distortion may be observed when, for example, the output power level in response to a first input power level may be pout1≈αpin1, where the output level in response to a second input power level may be pout2≈βpin2, when α≠β.
The AM-PM distortion provides a measure of the phase of the output signal in relation to the input signal (or output phase) in response to the input power level. Output phase is typically measured in units of angular degrees. The AM-PM distortion may be observed when, for example, the output phase changes in response to a change in input power level.
Limitations in the performance of PA circuitry due to distortion may be exacerbated when the PA is integrated in a single integrated circuit (IC) device with other radio frequency (RF) transmitter circuitry [such as digital to analog converters (DAC), low pass filters (LPF), mixers, and RF programmable gain amplifiers (RFPGA)]. Whereas the pressing need to increase the integration of functions performed within a single IC, and attendant increase in the number of semiconductor devices, may push semiconductor fabrication technologies toward increasingly shrinking semiconductor device geometries, these very semiconductor fabrication technologies may impose limitations on the performance of the integrated PA circuitry. For example, utilizing a 65 nm CMOS process may restrict the range of input power levels for which the PA provides linear output power level amplification.
The AM-AM distortion and/or the AM-PM distortion comprise transmitter impairments that may result in signal transmission errors that may result in unintentional and/or undesirable modifications in the magnitude and/or phase of transmitted signals. When transmitting quadrature RF signals, the AM-AM distortion and/or the AM-PM distortion may cause unintentional and/or undesirable modifications in the magnitude and/or phase of the I components and/or Q components in the transmitted signals.
The transmission of erroneous signals from an RF transmitter may result in erroneous detection of data contained within the received signals at an RF receiver. The result may be reduced communications quality as measured, for example, by packet error rate (PER), and/or bit error rate (BER).
Communications standards may specify a limit for Error Vector Magnitude (EVM) in a transmitted signal. For example, IEEE 802.11g standard for WLAN communications specifies that EVMdB for a 54 Mbps transmitted signal may be no greater than −25 dB. Thus, some conventional RF transmitters may be required to limit the peak power level for signals generated by the PA to ensure that the transmitted signals comply with EVM specifications. One potential limitation imposed by the reduced output power level is the reduced operating range in wireless communications. In this regard, the EVM specification may reduce the allowable distance between a transmitting antenna and a receiving antenna for which signals may be transmitted from an RF transmitter and received by an RF receiver, in relation to the operating range that would be theoretically possible if the RF transmitter were able to transmit signals at the maximum, or saturation, output power level that could be generated by the PA.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.